1. Field of the Invention
The invention relates to a process for the manufacture of a semi-transparent thermoplastic composition of a polyphenylene ether resin and a styrenic radial block copolymer wherein the processes comprises melt-mixing the polyphenylene ether resin in a powder form with the styrenic radial block copolymer. The composition of the invention may also contain polystyrene resin.
The invention also relates to articles formed out of the compositions made by the process of the invention.
2. Brief Description of the Related Art
In the food handling area, clamshell styled containers having an opaque bottom and a see-through top are in great demand for prepared foods. Busy consumers desire to purchase entrees at a local store or restaurant and take the meal home for the family meal. Reheating the food in a microwave oven is often the method of choice because of convenience and time and consequently the clamshell styled containers must have sufficient heat resistance to withstand steam and hot greases and oils. Additionally, the containers must have sufficient ductility to not break on handling and use by both the food preparer and the consumer.
In order to achieve great penetration into the clamshell styled container market, a resin needs to be of low cost and have outstanding processability, have sufficient transparency for food to be seen through the container top, have sufficient ductility to withstand breakage, and be able to resist hot greases and oils from foods on heating in a microwave oven.
Polystyrene resins are widely used thermoplastic resins because of their low cost and easy processability. These resins are available as homopolymer polystyrene (often called crystal polystyrene), as block copolymers with elastomeric materials (e.g., S-B-S, S-(EB)-S, S-EP copolymers), and as impact modified graft resins with polybutadiene resin (HIPS). Within these categories, there are flame-retardant, abrasion resistant, super-tough, UV-resistant, expandable, and environmental stress crack resistant grades that enable these resins to be used in a wide variety of everyday consumer goods such as toys, packaging, housewares, construction materials, automobile parts, and disposables.
Polystyrene resins are currently used in many clamshell designed containers, both in foamed opaque clamshells and in containers having see-through lids. Unfortunately, the properties of the various polystyrene resins are insufficient albeit for different reasons for great penetration into the market for clamshell styled containers having an opaque bottom and a see-through top. For example, crystal polystyrene is insufficient in ductility although acceptable in cost, processing, and transparency for the top. Addition of rubbery styrenic copolymers (S-B-S, S-EB-S) to crystal polystyrene improves the ductility but at the expense of the necessary transparency. High impact polystyrene (i.e., HIPS) has good impact strength but is not transparent. Polystyrene-butadiene radial block copolymers containing over fifty percent styrene exhibit good ductility, cost, and transparency but are insufficient in heat resistance.
Poly(phenylene ether) resins (referred to hereafter as "PPE") are commercially attractive materials because of their unique combination of physical, chemical, and electrical properties. Commercially, most PPE are sold as blends with predominantly high impact polystyrene resins. PPE are miscible with polystyrene resins in all proportions and because of the very high glass transition temperatures of PPE, the blends of PPE with polystyrene resins possess higher heat resistance than that of the polystyrene resins alone. Moreover, the combination of PPE with high impact polystyrene resins results in additional overall properties such as high flow and ductility, however, such blends are not transparent or even semi-transparent. Examples of such blends can be found in U.S. Pat. Nos. 3,383,435; 4,097,550; 4,113,800; 4,101,503, 4,101,504; 4,101,505; 4,128,602; 4,139,574; and 4,154,712 among others. The properties of these blends can be further enhanced by the addition of various additives such as impact modifiers, flame retardants, light stabilizers, processing stabilizers, heat stabilizers, antioxidants and fillers but none of these additives results in transparent or semi-transparent compositions that would be useful for the aforementioned clamshell styled containers having a see-through top. Moreover, the melt blending of styrene-butadiene radial block copolymer, wherein the styrene content in the copolymer is at least 50% by weight based on the weight of the copolymer, with pellets of PPE or PPE blended with high impact polystyrene results in degradation of the styrene-butadiene radial block copolymer and sheet made the composition has an unacceptable surface quality, including unacceptable gels.
It is therefore apparent there continues to be a need for improved compositions as well as processes to manufacture compositions containing polystyrene resins that have acceptable ductility, heat resistance, and transparency for use in clamshell styled containers having a see-through top.